The present invention relates to a precision machined article and a method of precision machining an article. More specifically, the invention relates to a precision machined optical wafer and a method of precision machining an optical wafer.
Due in large part to an increasing demand for very accurately machined parts, precise, automated machining has become a wide and diverse field of endeavor. The relationship between dimensional tolerance of machined parts and the life expectancy and performance of those parts was recognized early in the development of the art. Parts machined to tight tolerances generally offered better performance and were found to last longer. To meet the demand for precision parts, a machining art based upon sensors coupled to automatic machine controllers has developed.
In U.S. Pat. No. 4,954,022, Underwood et al., a workpiece of known shape is positioned on a moveable table beneath a movable tool. See col. 1, ll. 45-65. The tool is moved into electrical contact with the workpiece and the spatial location of the contact point relative to a reference position is recorded. The tool then bores a hole of preset depth into the workpiece. See col. 2. ll. 39-45. A computer compares the recorded point of contact to the expected point of contact. The expected point of contact is based upon the known shape of the workpiece and its known initial position on the table. The tool retracts, moves to a second point on the workpiece and makes electrical contact with the workpiece. The difference between expected point of contact is compared to the actual point of contact at each position where a hole is to be bored to insure uniform depth of the holes, ". . . the exact positions of the workpiece with respect to the expected positions of the workpiece are displayed to insure that each subsequent cut was made to the same uniform depth as the previous cuts in the workpiece." (Col. 2, ll. 21-25.) The ideal end product is a workpiece having a set of holes of uniform depth, each hole centerline having a preset angular relation to the other hole centerlines.
The machining proceeds in a stepwise way: the entire surface is not sensed and fitted prior to the start of machining operations.
Some drawbacks associated with this machining strategy are: an expected shape of the workpiece must be known and entered into the computer; precision positioning is required for both the tool and the workpiece; multiple degrees of freedom of both the tool and the workpiece are needed to complete the machining task; and, particular precautions must be taken to avoid a false electrical contact signal. These precautions include flushing the surface with a dielectric liquid, a step which is not compatible with some materials and processes. See col. 2, ll. 57-68.
An automated grinding method using sensor feedback throughout the operation is described in U.S. Pat. No. 3,665,652, Gordon et al. The method employs sensing means, a controller and a computer to determine ". . . the amount of material removed from the workpiece. " (Col. 1, l. 44). A comparison of sensor signals and D/A convertor signals yields ". . . the degree of out of round of the workpiece . . . " . (Col. 1, l. 39). Also, circuitry is in place". . . for generation of a signal representing the rate of removal of material from the workpiece." (Col. 1, ll. 46-48). This patent exemplifies a relatively complex sensing and controlling system. In general, as complexity of the automatic machining system and number of parameters controlled or monitored increases, cost and, in some cases, machine downtime may be expected to increase.
In U.S. Pat. No. 4,636,960, McMurtry, "measuring operations are needed before machining to determine the position of a surface to be machined and after machining to check the dimensions of the machined surface." (Col.1, ll. 26-29.) The initial position of the workpiece is determined relative to, "test surfaces on the table on or adjacent to the workpiece". (Col.1, ll. 49-51.) An additional reference is required. "The workpiece has a datum bore having an axis defining the workpiece datum in the X,Y directions. The datum position of the workpiece on the table is defined by distances X,Y,Z! between the axis and the table datum surfaces TX, TY, TZ." (Col.2, ll. 29-34.) (Numbers referring to figures have been suppressed.) Before machining begins, "the work offset see col. 2, ll. 29-58! and the probe offset see col. 3, ll. 3-17! have to be taken into account." The tool can then be directed to move to a target position, taking into account the probe and workpiece offsets.
The problem of relating a probe position signal to an actual tool axis position adds appreciable complexity to the machine and complication, time and expense to its operation. Two sets of reference surfaces are required, TX,TY,TZ and the datum bore, and two sensing operations, before and after machining are stipulated. The machine is made more expensive and complicated by the requirement that position of the workpiece relative to the support table and relative to the tool be known. Furthermore, the accuracy and repeatability of machining is necessarily determined by the accuracy and repeatability of the tool moving and table moving mechanisms.
U.S. Pat. No. 5,136,224, Matsumura, et al., describes a method of detecting and reducing to numerical data a model surface through use of a stylus which contacts the model. "A method of creating NC data for performance of machining in accordance with the profile of a model is available and includes tracing the surface of the model with a stylus by means of tracer control, digitizing the model surface profile by accepting stylus position data at a predetermined period T, and creating the NC data using the digitized data." The NC data may be used subsequently to machine a workpiece in conformance with the surface acquired from the model. Both position and velocity data are taken from the stylus and digitized. (Col. 1, ll. 57-60.)
Again, in this case a separate apparatus, i.e., a stylus is used to take data on the surface. Also, the complication of using velocity data is introduced. And, the surfaced measured corresponds with the final machined surface, not a surface to be machined.
The above examples show that increased accuracy is obtained at the expense of:
(i) increased time to complete a machining operation; and, PA1 (ii) increased complexity of the equipment used to model the surface and control the actual machining step. PA1 which does not require knowledge of the workpiece surface shape prior to locating the surface with a sensor/detector arrangement; PA1 in which position data is referenced to a single reference position of the tool so that precision holding or chucking of the workpiece is not required; and, PA1 which automatically takes into account tool wear by using the tool as the probe. PA1 a) holding the article in place relative to a reference position of the cutting tool, PA1 b) moving the cutting tool into contact with the surface of the article, PA1 c) detecting the position of contact of the tool with the surface, PA1 d) recording the position of the contact of the tool with the surface, PA1 e) retracting the tool from the surface, PA1 f) moving the tool to a different position on the surface, PA1 g) repeating steps b) through f) at multiple positions on the surface, and PA1 h) machining at least one cut into the surface, wherein the machining is controlled by control means using positions recorded in the recording step. PA1 moving a detector to detect light emanating from light paths embedded in the wafer, and PA1 recording the location of the detector relative to the cutting tool reference position.
Precision machining difficulties are compounded when the shape of the workpiece is irregular or when the amount of material to be removed from the workpiece is small. For example, optical wafers, which incorporate embedded optical waveguide paths, can be machined to create a surface for securing an optical fiber pigtail whose endface is abutted to the embedded path. In a manual prior art process, labor intensive and time consuming steps are required to achieve acceptable alignment of the fiber pigtail and the embedded waveguide path. First, the wafer is cut into several pieces to reduce the deviation from datness within a wafer piece. The individual pieces are attached by means of wax to flat substrates in preparation for machining. While the wax hardens, the operator adjusts the position of the wafer piece manually in an attempt to place the piece in a horizontal plane relative to the downward direction of movement of a tool. The piece is then machined by moving the tool through the wafer piece while maintaining the vertical position of the tool. Even with exceptional care in segmenting the wafer and aligning the wafer pieces relative to a horizontal plane beneath the tool, machining results are sometimes unsatisfactory. In FIG. 8, the edge view of an optical wafer 36 shows varying distances between the machining line 40 and the location of the embedded waveguide paths 38.
There is, therefore, a need for a precision machining method which is lower in cost, simpler in concept and operation, and which is able to accept a wider range of workpiece sizes and shapes. More particularly, there is a need for a precision machining system which does not require pre-knowledge of the surface shape and which does not require precise reference points or surfaces for both tool and workpiece. Also, a precision machining strategy which automatically takes into account tool wear, is a desirable machining method enhancement.